Two degree-of-freedom (2-DOF) joint modules used in robotic arms are becoming more common due to several advantages such as: compact size, light weight and lower cost. Joint modules are designed to meet certain requirements and constraints and these are transformed into the design specifications. For industrial applications, the requirements of payload range, speed, accuracy, reliability, lifetime, safety, ease of assembly and maintenance are very important.
There is a type of 2-DOF joint module, called Powerball ERB™, designed by Schunk GmbH & Co. KG. This joint module is housed in a ball shape enclosure that contains all the components needed to control the joint: servo motor, encoder, motor drive, harmonic drive, holding brake and hollow shaft for internal cabling. The joint module is not sealed as ventilation is needed to dissipate heat generated by the electronic components such as motor, motor drive and brake. The module is light weight, compact and is highly integrated. However, this design has limitations.
First, the Powerball ERB™ joint module consists of many mechanical and electronic components and this increases the complexity of the structure while also creates a heat dissipation problem. Since all electrical and control components are integrated in the module housing, the heat generated by these components requires a relatively large space to dissipate. However, since this joint is designed to be a compact joint, the power consumed by the electronic components is constrained by the heat that is generated. This in turn limits the output power of the joint module. Hence, the application of this type of joint module in terms of payload range is limited.
Second, to solve the issue of heat dissipation, openings or slots are made on the housing. This limits the applications of the joint module under certain harsh industrial environments such as dusty, humid, and explosive environments. These joints could not be used in robot arms for painting, coating and welding. For example, the explosive gases and sparks that may be present in such industrial applications could get into the joint module and cause explosions.
Third, the Powerball ERB™ can be used to build a robotic arm, LWA-4P™. The LWA-4P arm comprises three Powerball joint modules and two links. Since the joint modules have limitations on heat dissipation and power capped issues, the arm cannot work under some harsh industrial environments and the payload of the arm is limited.
There is another 2-DOF joint module, designed by Engineering Services Inc. (ESI) with U.S. Pat. No. 9,044,865. This joint module is designed for large torque and low speed applications. The joint module includes a module housing and two joints. Also, one of joints has a hollow shaft gearhead, an off-axis drive, a servo motor, and internal cables extending through the hollow shaft gearhead. Since the joint module is designed to connect with a link, it has an active side and a passive side with electronic connectors. The active side is mechanically connected to the link and the electronic connectors of the passive side are operably connected to the link cables. The joint is used to build a robotic arm. There are limitations with this design as discussed below.
First, since all the components needed to control the motion of the joint are integrated into the module, it has the same heat dissipation problem mentioned in the Powerball EBR™.
Second, the cable routing inside the module is complicated because of the internal structure of the joint module. One of joints uses a non-hollow shafted motor and gearhead for providing the torque. Because of the internal structure of the joint, the cables go into one end of the module and inside the module turn 90 degrees and go out the other side of the module. In this case, the cables will be squeezed inside the housing. This may cause large torsional forces on the cables.
There is another type of 2-DOF joint module, designed by Fanuc Robotics North America as shown in U.S. Pat. No. 5,293,107. Each module housing accommodates two hollow shafted rotary actuators, other electronic components and internal cables. The joint is used to build a robotic arm. However, this design also has limitations.
First, the installation process of rotary actuators and electronic components is complicated because it requires too many assembly steps. The two actuator sets are installed inside the housing, with their output shaft facing outside and the motor facing inside of the housing. The two actuators will be fixed to the housing wall by bolts and screws. To mount the two actuators in the housing, the two actuators cannot be put in from outside to inside of the housing. Instead, the actuators must be installed from the inside. So, the entire housing must be dissembled. Once the actuators are installed the housing is reassembled as one piece with screws and bolts. Therefore, the installation process is complicated.
Second, the joint module housing of Fanuc is not made of one piece. The housing box is made of several pieces and these pieces are fixed by screws and bolts to form the housing. So, the structure of the housing is not as strong as the one-piece housing.
Third, the maintenance process of the joint module is complicated. To access the actuators and other electronic components, a user needs to dissemble the housing case, conduct the maintenance, and resemble the housing once the maintenance is finished.
All of the aforementioned approaches to modular joints have limitations for industrial applications. It would be advantageous to design a new type of 2-DOF joint module which will have features such as compact, low heat generation, sealed and rigid housing, large payloads, ease of installation and maintenance process and assembly.